Introduce a test for `cp'.

Hi,

I'm really keen on dabbling in Rust, and this project looks like a great one so I thought I would give it a try. Since this is my first attempt, I didn't want to bite off more than I can chew, so I've tried to start reasonably small by adding a test for `cp`. If this is good, I can expand it to get more coverage before adding any new features to make it more like the GNU version.

Please do let me know your thoughts, all feedback is greatly welcomed.

Thanks,
Jay
I don't think the `.into_string()` is necessary here.